Gabbroic layers and pockets are commonly found in upper mantle-derived peridotites (e.g., gabbros in the Horoman peridotite complex, Japan; mafic layers in the Ronda peridotites). There is a general consensus that gabbros in association with the mantle-derived peridotites do not show any co-genetic relationships in chemical compositions as residues and the partially fused melts. Effects of small melts on the modification of the gabbros as well as peridotites were investigated in this research.(1) H2O-undersaturated solidus of MORB pyrolite locates at a pressure of 28 kb at T=1000ﾟC and at a temperature of 1075ﾟC between P=18 and 23 kb. This solidus coincides with the stability limit of pargasitic amphibole in MORB pyrolite composition and the location of solidi can be controlled by the Na2O+K2O content of peridotites.(2) 10 kb H2O-undersaturated solidus of the Horoman gabbro was determined near T=950ﾟC from the sandwich-type melting experiments. The maximum temperature stability of pargasitic amphibole in gabbros occurs at T=1075ﾟC under P=10 kb, which is approximately 50ﾟC higher than that for MORB pyrolite. It is clear that the amphibole can be an agent controlling the decomposition and modification on hydrated peridotites under the P-T conditions near solidus.(3) Effect of small amounts of melt on peridotites is observable near magma channels in the upper mantle-derived peridotites, where a replacement can be generated during small degree of partial melting of the wall peridotites as well as melt addition from the channel.